Principled Metalic shader in 2.79RC2 is not influenced by the Specular value. Bug or feature?
This is the correct behaviour. 

In the principled shader the specular value is used to derive the index of refrection (IOR) used for reflection on the material. However, for metals, the reflectivity/IOR is instead determined from the surface colour, so the specular value here should be ignored.
Th paper it is based on can be found here. In particular section 5.2 describes the parameters, noting that for the metallic model, the specular is derived from the base colour:


  The metallic model has no diffuse component and also has a tinted
  incident specular, equal to the base color


While not exactly the same as the principled bsdf, the unreal engine's PBR shader is also based on the same paper by disney, and uses similar parameters, their description is worth a read.